Method for scheduling wake/sleep cycles by a central device in a wireless network

ABSTRACT

Methods and systems for scheduling wake/sleep cycles by a central device in a wireless network that have at least one mobile device include determining a system reference cycle as a minimum value of a delay constraint on a real-time service for each mobile device of the at least one mobile device that has a real-time service. A wake/sleep cycle length is attributed to each mobile device. The wake/sleep cycle length is an integer multiple of the system reference cycle such that a wake/sleep cycle length of a first mobile device is different from that of a second mobile device. A sleep period and a wake period are assigned within the wake/sleep cycle of each mobile device. The wake/sleep cycle of each mobile device is arranged to avoid collision of the wake period with those of other mobile devices.

FIELD OF THE INVENTION

The present invention relates generally to the power management in awireless network, and more particularly, to a method for schedulingwake/sleep cycles by a central device in a wireless network.

BACKGROUND OF THE INVENTION

A major constraint for many mobile applications (e.g., mobile TV, VOD(Voice on Demand)) is the limited capacity and lifetime of the batteriesof mobile devices. It is reported that in a small-size mobile devicelike a PDA (Personal Digital Assistant), the percentage of power drainedby the wireless interface is up to 50% of the overall systemconsumption. Without a power management module on the wirelessinterface, the energy of a mobile device can be drained out quickly.Therefore, energy management of wireless interface has become animportant issue.

In view of the above issue, a sleep mode was proposed for wirelessnetworks, in which ideally a mobile station (MS) will power down itswireless interface with a base station (BS) to enter into a sleep statewhen there is no data for it to receive or transmit, and wake up onlywhen there is data for it. The sleep mode intends to minimize MS powerconsumption and decrease usage of BS air interface resources. It is alsoa main task of the energy management to schedule the state (i.e. sleepor wakeup) transition of the wireless interface of the MS in order tominimize its energy consumption because the state transition betweensleep and wakeup will also consume energy. In order to reduce thefrequency of state transitions, a solution was proposed for the sleepmode to buffer and deliver data in a burst manner, for example, by theslicing technique of DVB-H.

FIG. 1 is an exemplary diagram showing a scheduling method with a burstmanner for power-saving of a MS in wireless network in the prior art.FIG. 1(a) shows the sleep mode with an immediate transmission manner forcomparison with the sleep mode with the burst manner shown in FIG. 1(b).

As shown in FIG. 1(a), the MS enters into a sleep state in sleep timeslots and wakes up to send or receive data packets in wakeup time slotsindicated by the pulse parts in the FIG. 1(a). Now referring to FIG.1(b), a periodic sleep mode with the burst manner is shown, in which asleep cycle period for the MS is divided into a sleep window and alistening window. In the sleep window, the MS can power off thecorresponding wireless interface or puts the wireless interface at a lowpower level. In the listening window, the MS wakes up, i.e., powers onthe wireless interface to receive and/or send its data packets. Packets,arriving at or destined to the MS, are buffered and then transmitted ina burst manner within the listening window.

Before entering into the periodic sleep mode, the MS negotiates with theBS about the length (in the units of the physical (PHY) frame) of thelistening window, the length of the sleep window, and the starting PHYframe from which the MS starts the periodic sleep cycle. As shown inFIG. 1(b), the sleep window can be set as the maximum packet delay ofthis MS.

Comparing with the immediate transmission manner, which is shown in FIG.1(a), the sleep mode with the burst manner can obviously reduce thefrequency of state transitions, which will in turn reduce the powerconsumption. But on the other hand, this solution will lead to longerpacket delay. The trade-off between the power-saving and the packetdelay for IEEE 802.11 network was already studied.

However, existing scheduling methods for IEEE802.11 power management cannot readily satisfy the objective of saving power and maintaining QoS(Quality of Service) guarantee simultaneously in such wireless networkas IEEE802.16e, where QoS requirements are explicitly specified.

In addition, for IEEE 802.16e systems, existing research only focuses onadaptive sleep mechanisms for web browsing service and on single-MSenvironments. However, in practical operation, there is usually morethan one MS in the regime of a BS.

As described above, the sleep mode is used in wireless networks forpower saving of the MSs. Full information regarding the sleep mode isgiven in the IEEE standard “IEEE802.16e-2005”.

In a wireless network having multiple MSs associated with a BS, thetransmission of traffic of these MSs will be influenced from each otherbecause the system radio resource is shared among all these MSs insteadof being dedicated to one MS among them. FIG. 2 is a diagram showingresource collision in a multi-MSs wireless network in the prior art. InFIG. 2, three mobile stations MS1, MS2 and MS3 are shown, assigned withthe sleep cycle periods of 5, 11 and 18 time slots respectivelyaccording to the traditional power-efficient scheduling method for asingle-MS environment. In each sleep cycle period, the pulse partsdenote the time slots of the listening window for this MS. The sleepcycle period is generally less than the maximum packet delay requirementof an application running on the MS. Otherwise, the QoS requirements ofthe service cannot be guaranteed.

As shown in FIG. 2, under the time divisional protocol, one time slotcan only be allocated to one MS as the listening window. If the timeslot 10 is allocated to MS1 as listening window to satisfy itspower-saving schedule, the schedule of MS2 cannot be satisfied at thesame time. Otherwise, a resource collision will take place for time slot10, as marked by black parts in FIG. 2. This kind of collision alsotakes place at time slots 35, 54 and 55. Therefore, the design of a goodpower-saving scheduling algorithm with Quality of Service (QoS)guarantee for multi-MSs environment is of more practical importance butalso more complex.

For the IEEE 802.16e network, several scheduling approaches wereproposed to carry out a power-saving schedule of multiple MSs and at thesame time maintain the QoS guarantee.

In an approach 1 described by a paper “Improving mobile station energyefficiency in IEEE 802.16e WMAN by burst scheduling, G. Fang, E.Dutkiewicz, Y. Sun, J. Zhou, J. Shi, Z. Li, IEEE Globecom, 2006”, the MSthat has the shortest time to reach its maximum bit rate requirement isselected as the primary MS. The scheduler of the BS allocates almost allthe bandwidth in a burst to the primary MS and allocates just enoughbandwidth to other awake-state MSs to guarantee their minimum data raterequirements.

This approach 1 does not take into consideration real-time services thathave packet delay constraints (there is no such constraint fornon-real-time services). Some studies show that this approach cannotconserve energy efficiently for TV-like multicast services having staticperiodic schedule pattern. Besides, it requires a lot of signalingexchanges, which will not only cost bandwidth but also introducesignalling transmission delay. Please note that the types of datadelivery services, including the real-time service and non-real-timeservice, are defined in the above mentioned IEEE801.16-2005 standard,where full information concerning the definition and requirements ofeach type of services is given.

A paper “Energy efficient integrated scheduling of unicast and multicasttraffic in 802.16e WMANs, Lin Tian, et. al., IEEE GLOBECOM 2007”described an approach 2 that proposed to firstly allocated resources toreal-time multicast services in a periodic burst manner to save power.In this approach 2, remaining resources are allocated to non-real-timeunicast services in an order that resources are firstly allocated to themulticast-group-member MS and then to the MS that only have unicastservices. The resource allocated to the unicast service of amulticast-group-member MS is adjacent to the resources for its multicastservice. FIG. 3 is an exemplary diagram showing the power-efficientscheduling method of the approach 2. FIG. 3(a) shows resources initiallyallocated to the unicast and the multicast services for MS1 to MS4respectively. FIG. 3(b) shows resources allocated to the unicast and themulticast services of each MS are allocated to be adjacent to eachother. In fact, the approach 2 assumes that the delay constraints of allthe real-time multicast services are the same. It cannot be extended tothe general environment where multiple real-time services with differentdelay constraints (e.g., VoIP and video) exist.

According to an approach 3 in a paper “Energy Efficient Scheduling withQoS Guarantee for IEEE802.16e Broadband Wireless Access NetworksShih-Chang Huang, Rong-Hong Jan., Chien Chen, (2007 IWCMC)”, in a casethat multiple MSs have real-time services with different delayconstraints, a common sleep cycle period is determined by choosing theminimum delay constraints among all services of the MS. All MSsperiodically sleep and wake up to receive their data with the commonsleep cycle period. FIG. 4 shows the power-efficient scheduling methodof the approach 3. As shown in FIG. 4, the common sleep cycle period inthis example is 6 frames long for five MSs A, B, C, D and E. The BScomputes the number of frames needed for the MSs it serves. Since oneframe slot can only be allocated to one MS, the BS schedules MS E in the1^(st) frame, and delays the starting times of MS D, C, B and Arespectively to the 2^(nd), 4^(th), 5^(th), and 6^(th) frames withineach sleeping cycle period. In this way, the allocated frames can bescheduled without overlapping, that is, without resource collisionissue.

The advantage of the approach 3 is that it has a simple schedulingalgorithm. However, with a common scheduling cycle for all MSs, a MShaving larger delay constraints and thus having larger sleep cycleperiod than the common cycle period will apparently have to perform thestate transition more frequently than it is scheduled in the single-MSenvironment. As described above, state transition between sleep andwakeup will also consume a large amount of energy, which is generallymore than one slot unit of energy consumed in wakeup state. Therefore,approach 3 will lead to more energy consumption for the MSs with delayconstraints that are larger than the common scheduling period.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method for schedulingwake/sleep cycles by a central device in a wireless network is provided.The wireless network comprises at least one mobile device. The methodcomprises: attributing a wake/sleep cycle length to each mobile device,wherein the wake/sleep cycle length is an integer multiple of a systemschedule cycle; assigning a sleep period and a wake period within thewake/sleep cycle of each mobile device; and arranging the wake/sleepcycle of each mobile device to avoid collision of the wake period withthose of other mobile devices.

According to one aspect of the invention, a method for schedulingwake/sleep cycles of a mobile device is provided. The method comprisesthe steps of: receiving from a central station scheduling data for asleep/wake cycle, wherein the sleep/wake cycle length is defined as aninteger multiple of a system schedule cycle; and waking and setting tosleep the appropriate circuits as a function of the scheduling data.

BRIEF DESCRIPTION OF DRAWINGS

These and other aspects, features and advantages of the presentinvention will become apparent from the following description inconnection with the accompanying drawings in which:

FIG. 1 is an exemplary diagram showing a scheduling method with a burstmanner for the power-saving of a MS in wireless network in the priorart;

FIG. 2 is a diagram showing resource collision in a multi-MSs wirelessnetwork in the prior art;

FIG. 3 is an exemplary diagram showing a power-efficient schedulingmethod in the prior art;

FIG. 4 is an exemplary diagram showing another power-efficientscheduling method in the prior art;

FIG. 5 is an exemplary diagram showing an embodiment of the schedulingmethod in accordance with the principle of the present invention;

FIG. 6 is a flow chart showing the work flow of one embodiment of apower-saving scheduling method; and

FIG. 7 is a flow chart showing the work flow of a method to adjust sleepcycle periods of the MSs.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, various aspects of an embodiment of thepresent invention will be described. For purposes of explanation,specific configurations and details are set forth in order to provide athorough understanding. However, it will also be apparent to one skilledin the art that the present invention may be practiced without thespecific details present herein.

In view of the disadvantages of the above approaches, a power-savingscheduling method for multiple MSs with various types of services in awireless network is provided in accordance with an embodiment of thepresent invention. According to the general concept of the method, arelatively static service schedule pattern is firstly designed to reducepower consumption of MSs having real-time services and satisfy theirminimum QoS requirement. Because the minimum QoS rate and maximum delayof all MSs having real-time services are negotiated early at eachservice establishment, it can be regarded as pre-known traffic from theviewpoint of the service scheduler. Further, the traffic that isnon-predictable (e.g., traffic produced by web browsing and FTPdownload) is scheduled on the fly to fully utilize the remaining systemresources and to achieve fairness and reduced power consumption at thesame time.

For the scheduling of MSs having real-time services, there is theresource collision problem, as described above in FIG. 2. According tothe method of the embodiment, a system reference cycle period is firstlydetermined. Then the scheduling cycle period of each MS is set as aninteger multiple of the system reference cycle period. A sleep windowand a wake window are assigned within the wake/sleep cycle of eachmobile device and also the scheduling cycle period of all the MSs arearranged to avoid collision of the wake windows among each other.According to the solution, at the beginning of scheduling, the startframe of sleep mode of each MS is adjusted within one system schedulingcycle in order to avoid the potential resource collision. That is, oncethe resource collision problem is settled at the beginning of thescheduling, no more scheduling is need in the subsequent schedulingcycles in view of the resource collision.

In this embodiment, the system reference will be less than the minimumvalue of delay constraints of all MSs having real-time services. The“delay constraint” of a MS may be defined as the maximum delayconstraint among all the real-time services running on the MS.

The scheduling cycle period of a MS can be set as large as possible butless than its maximum delay constraint. Additionally, each MS can be setto keep its own sleep window as large as possible if the wake windowwill not collide with those of the other MSs.

FIG. 5 shows an embodiment of the scheduling method in accordance withthe principle of the present invention, which aims to keep the maximumsleep cycle period of each MS to guarantee the QoS requirement and toefficiently utilize the system bandwidth.

A scheduling for three MSs, which is similar to the case of FIG. 2, isshown in FIG. 5(a). As shown in FIG. 5(a), the sleep cycle periods ofthe three MSs are set to be integer multiples of a system schedulingcycle, for example 5 time slots in this case. As one example, the sleepcycle period of MS1 is set as 5 time slots, which is the same as that inFIG. 2. The sleep cycle periods of MS2 and MS3 are changed from 11 and18 time slots which is shown in FIG. 2 as prior art to 10 and 15 timeslots respectively in accordance with the embodiment of the invention.

In the above embodiment, the minimum value of maximum delay constrainsamong all the MSs, 5 time slots in this case, is set as the systemscheduling cycle period. The scheduling cycle period of each MS is themaximum integer multiple of the system scheduling cycle period but isless than its maximum delay constraint. Accordingly, the listeningwindow for receiving QoS guaranteed traffic of each MS can becalculated. Details on how to select the system scheduling cycle and thescheduling cycle period of each MS will be described later withreference to FIG. 6.

Then, as an example, in the first minimum scheduling cycle, the startframe of each MS's sleep cycle period is arranged to avoid resourcecollision. As a result, there will be no resource collisions in thefollowing scheduling cycles.

According to this embodiment, if one more station, MS4 as shown in FIG.5(b), enters the system, the scheduling scheme in FIG. 5(a) might not beable to accommodate the new traffic. In order to fully utilize thesystem bandwidth, or alternatively, to maximize the system capacity,some MSs having larger sleep cycle period can be shortened. In FIG.5(c), as an example, the scheduling cycle period of MS2 is shortenedfrom 10 frames to 5 frames, and correspondingly its listening window isshortened from 2 frames to 1 frame. As a result, one frame of resourceis released by MS2 for arranging MS4 in the first system cycle period inFIG. 5(c). With this minor change to the existing MSs and setting thescheduling cycle period of new corner MS 4 to 5 frames, the traffic ofMS4 can be accommodated into the system and meanwhile the powerconsumption of these MSs can be reduced. The MS2 that originally haslarger sleep cycle period is reduced in this period for the MS4 to jointhe system, by which the system resources are fully and fairly utilizedand at the same time power consumption can also be reduced.

According to one aspect of the present embodiment, the sleep cycleperiod of each MS can be kept as long as initially determined. Inaddition, the signalling overhead can be kept at a reasonable level. Thesleep parameters (sleep period, listening window, and start frame) forreal-time services are exchanged between the scheduler and each MS onlyonce, which introduces minimum amount of signalling overhead.

FIG. 6 is a flow chart showing the work flow of one embodiment of apower-saving scheduling method. For the convenience of explanation, themeaning of the notations used hereafter is given in Table 1. Differenttypes of services and their QoS requirements are given in Table 2.

TABLE 1 definition of notations notation Meaning M Total number of MSshaving power-saving requirement m Index of MSs, m ∈ [1, M] x Index ofservice types. x = 1, 2, 3, 4 correspond to UGS, rtPS, nrtPS, BEservices respectively y Index of connection C_(m,x,y) yth Connection ofservice type x of MS m R_(m,x,y) ^(min) Minimum reserved bit rate forthe yth connection of service type x of MS m R_(m,x,y) ^(max) Maximumsustainable bit rate for the yth connection of service type x of MS mD_(m,x,y) Maximum packet delay constraint for the yth connection ofservice type x of MS m ξ_(m,x,y) Tunable delay threshold of C_(m,x,y);ξ_(m,x,y) ∈ (0, D_(m,x,y)). The emergency data is the data whose waitingperiods exceed ξ_(m,x,y). d_(m,x,y) Maximum delay of the packets inbuffer of C_(m,x,y). T_(m,x,y) ^(base) Time base used to measure thetraffic rate. E.g., the resources allocated to connection C_(m,x,y)shall not be less than T_(m,x,y) ^(base) · R_(m,x,y) ^(min) in theinterval of T_(m,x,y) ^(base). It is in the magnitude of second. Ω_(k)Capacity (bits) of a resource unit. k ∈ [1, K] depending on selectedmodulation & coding scheme (MCS). There are K types of MCSs. T_(frame)PHY frame duration E^(L) Energy consumed per resource unit when a MS isin wake state E^(T) Energy consumed by sleep-to-wake state transition

TABLE 2 Service types and their QoS parameters defined in IEEE 802.16Service Type QoS Parameters Typical service Real-time UGS (UnsolicitedGrant Service) R_(m,1,y) ^(min), D_(m,1,y) MPEG-2, CBR service rtPS(real-time Polling Service) R_(m,2,y) ^(min), R_(m,2,y) ^(max),D_(m,2,y), T_(m,2,y) ^(base) VolP None-real- nrtPS (non-real-timePolling R_(m,3,y) ^(min), R_(m,3,y) ^(max), T_(m,3,y) ^(base) FTP timeservice Service) BE (Best Effort service) R_(m,4,y) ^(min) = 0,R_(m,4,y) ^(max) Email, etc.

Firstly, all the MSs in the regime of a BS are classified into two setsaccording to the priority of services they have. In this embodiment, asan example, a real-time service has a high priority and a non-real-timeservice has a lower priority. Thus one set, denoted as A, comprises MSsthat have real-time services and the other set, denoted as Ā, includesthe remaining MSs. A relatively static service schedule pattern can bedesigned to reduce power consumption of MSs in set A and satisfy theirminimum QoS requirement (or “QoS Rate”). Further, the “non-QoS rate”traffic that is non-predictable is scheduled on the fly to fully utilizethe remaining system resources and to achieve fairness and reduced powerconsumption at the same time.

Here, “QoS Rate” is defined as the instantaneous rate requirements forQoS guarantee:R _(m,x,y) ^(q)=Min{Min{R _(m,x,y) ,R _(m,x,y) ^(max)},Max{R _(m,x,y)^(em) ,R _(m,x,y) ^(min)}}

where R_(m,x,y) ^(em), R_(m,x,y) denote the rate to transmit theemergency data, and the rate to transmit all buffered data for theconnection C_(m,x,y). The emergency data is the data whose waitingperiod exceeds ξ_(m,x,y). Accordingly, the “non-QoS rate” of aconnection is defined asR _(m,x,y) ^(nq) =R _(m,x,y) −R _(m,x,y) ^(q)

The “non-QoS rate” traffics include parts of rtPS traffic, and all thenrtPS and BE data.

Then, the method firstly proceeds to a static scheduling stage for theMSs having real-time services which comprises the following steps of:(a) determining a system scheduling cycle; (b) determining the sleepcycle period of each MS and the listening window in order to keep thesleep cycle periods as large as possible and to avoid resource collisionas well; and (c) adjusting the start time of each MS's sleep mode in thefirst system scheduling period to avoid resource collision.

As one example of the step (a), the system scheduling cycle can be theminimum delay constraints among all MSs in set A, as given in thefollowing Equation (1).T ^(S)=min{T _(m) ^(C) ,m∈A}  (1)

As for the step (b), the sleep cycle period of each MS and the listeningwindow can be given by the following Equations (2) and (3).

The scheduling cycle of a MS having real-time services, T_(m) ^(S), isdefined asT _(m) ^(S) =[T _(m) ^(C) /T ^(S) ]·T ^(S) ,m∈A  (2)

The scheduling cycle of a MS is used as the sleep cycle period of the MSin set A. The listening window of the MS in A, T_(m) ^(L), is definedas:T _(m) ^(L)|(Σ_(x=1) ³Σ_(y) R _(m,x,y) ^(min))·T _(m) ^(s)/Ω₁ |=[R _(m)^(min) ·T _(m) ^(s)/Ω₁ ],m∈A  (3)

In Equation (3), the number of frames allocated to a MS is calculatedbased on the minimum capacity of a frame. This gives room to theadjustment and scheduling of the unpredictable traffics because thecapacity of a frame can increase when the channel condition between theMS and the BS becomes better. Besides, since the periodic wakeup of theMSs having real-time services (UGS, rtPS) is inevitable, it is better tofully utilize each listening window to also transmit the “QoS rate” ofnrtPS connections.

Correspondingly, the sleep window of the MS is:T _(m) ^(I) =T _(m) ^(S) −T _(m) ^(L)(frames)

As described above, the start time of each MS's sleep mode in the firstsystem scheduling period is adjusted by the step (c) to avoid resourcecollision. Since there is no collision in the first system schedulingperiod, there will be no collisions in all the following schedulingcycles.

Specifically, the sleep cycle period of a MS having real-time services,T_(m) ^(C), is defined asT _(m,x,y) ^(C)=└ξ_(m,x,y) /T _(frame) ┘,x∈[1,2](frames)T _(m) ^(C)=min{_(y∈[1,T) _(m,UGS]) ^(min) {T _(m,1,y) ^(C)},_(y∈[1,Y)_(m,rtPS]) ^(min) {T _(m,2,y) ^(C)}}

According to the equation (1), the system scheduling cycle can be theminimum delay constraints among all MSs in set A. As an alternative, theoptimum value of system scheduling cycle period and each MS's schedulingcycle period can be obtained by jointly optimizing a power efficiencyfunction. Specifically, let the scheduling cycle of a MS m, T_(m) ^(S),be integer multiple of the system scheduling cycle period under theconstraint of being no larger than the independent sleep cycle period ofthe MS. That is, mathematically,T _(m) ^(S) =n _(m) ·T ^(S)  (4)

where the integer n_(m) satisfies 1≤n_(m)≤└T_(m) ^(C)/T^(S)┘.

Obviously, when n_(m)=└T_(m) ^(C)/T^(S)┘, Equation (5) equals toEquation (2). The power efficiency function ƒ(T^(s),{right arrow over(n)}) has two variables, T^(s) and a variable set {right arrow over(n)}, {right arrow over (n)}={n_(m), m∈[1, M_(A)]}. The optimum systemscheduling cycle period T^(S)* and the optimum scheduling cycle periodof all MSs {right arrow over (n)}* are obtained by maximizing the powerefficiency function as follows:

$\begin{matrix}\begin{matrix}{\left( {T^{S^{*}},{\overset{\rightarrow}{n}}^{*}} \right) = {{\arg\mspace{11mu}\max\mspace{14mu}{f\left( {T^{S},\overset{\rightarrow}{n}} \right)}} = {\arg\mspace{14mu}{\max\left( {\sum_{m = 1}^{M_{A}}\;\left( {T_{m}^{S} - T_{m}^{L}} \right)} \right)}}}} \\{= {\arg\mspace{11mu}{\max\left( {\sum_{m = 1}^{M_{A}}\left( {{n_{m} \cdot T^{S}} - \left\lceil {R_{m}^{\min} \cdot n_{m} \cdot {T^{s}/\Omega_{1}}} \right\rceil} \right)} \right)}}}\end{matrix} & (5)\end{matrix}$

So long as total traffic of the MSs is within the system capacity, anoptimum solution pair can be obtained by solving Equation (5).Enumeration is a simple way to find the optimum solution. But when thevariable space is large, the enumeration method can becomputation-consuming, and some heuristic algorithms such as Equations(1), (2) and (5), may be used alternatively.

In the method as shown in FIG. 6, when the number of MSs is increasedbecause of joining of new MSs, the sleep cycles of the MSs having largesleep cycle periods, set in the steps of (a) and (b), can be shorten tofully utilize the system bandwidth to accommodate more MSs. One exampleof algorithm to adjust the sleep cycle is shown in FIG. 7, which will bedescribed in more detail later.

Next, the method proceeds to the on-the-fly scheduling stage in whichthe scheduler allocates the remaining sporadic resources to “non-QoS”traffics. The on-the-fly scheduling includes following steps of: (d) ifthe frame has not been allocated in the static stage but its adjacent MSis in set A and has data waiting to transmit, allocating the frame toits adjacent MS according to the priority of services and then to thefairness principle; and if the fairness index is higher than athreshold, releasing the frame for other MSs' non-QoS traffics waitingto be transmitted; (d) allocating the frame to non-QoS traffic of rtPS,nrtPS and BE services according to priority, fairness index and MS'schannel condition; if no traffic waiting to be transmitted, leaving theframe un-allocated; (e) when there are several MSs in set Ā havingservices of equal conditions (e.g., priority), allocating the resourceto the MSs having good channel conditions. The rationale behind is that,if the transmission error is high, it is actually a waste of power of MSto wake up to receive the data.

Referring now to FIG. 7 which shows the work flow of a method to adjustsleep cycle period of the MSs having larger sleep cycles in order tofully utilize the system bandwidth to accommodate more MSs andsimultaneously to let each MS keep its sleep cycle as longer aspossible. Specifically, if the listening windows of each MSs in set Acan not be scheduled without resource collision in one system schedulingcycle period, as shown in Eq. (4), then the sleep cycle period of someMSs should be shortened.Σ_(m=1) ^(M) ^(A) T _(m) ^(L) >T ^(S)  (6)

where M_(A) denotes the number of MSs in set A.

For the adjustment of the sleep cycle period of the MSs, severalproblems should be considered, for example, which MS(s) will beinfluenced and how many frames long should their sleep cycle period be?Different choices may result in different power-saving performance. Inthe algorithm shown in FIG. 7, the sleep cycle period is shortened byone system scheduling period first, then two system scheduling periods,and so on. In each iteration of sleep cycle period adjustment, the MSshaving larger listening window will be considered firstly. The MS, whichcan release one unit of resource (e.g., one frame) when its sleep periodis shortened, will be chosen firstly to change its sleep period, and theMS which can not release one unit of resource will keep its sleep periodunchanged. The iteration will stop when all MSs can be accommodated bythe system. If the sleep period of all MSs in set A have been shortenedto the system scheduling period and the condition in Equation (4) stillcan not be satisfied, the system will have to reject the new cornerMS(s).

It is to be understood that numerous modifications may be made to theillustrative embodiments and that other arrangements may be devisedwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

The invention claimed is:
 1. A method for scheduling power save cyclesby a central device in a wireless network comprising a plurality ofwireless devices, said method comprising: determining a system referencecycle as a minimum value of a delay constraint on a real-time servicefor each wireless device of said plurality of wireless devices that hasa real-time service; attributing a length of a power save cycle to eachwireless device, wherein each power save cycle length is an integermultiple of the system reference cycle and wherein the integer value isvaried to form a difference between the power save cycle length for eachof the plurality of wireless devices; assigning a sleep period and awake period within the power save cycle of each wireless device usingthe predetermined power save cycle length; and arranging the power savecycle of each wireless device to avoid collision of the wake period withthose of other wireless devices.
 2. The method according to claim 1,wherein the step of arranging the power save cycle of each wirelessdevice comprises adjusting the starting time of the power save cycle ofeach wireless device to avoid collision of the wake period with those ofother wireless devices.
 3. The method according to claim 1, wherein oneor more wireless devices have a service with a high priority.
 4. Themethod according to claim 3, wherein a real-time service has the highpriority.
 5. The method according to claim 4, wherein the power savecycle length of a wireless device having real-time service is attributedas the maximum integer multiple of the system reference cycle under theconstraint of being no larger than the maximum delay requirement onreal-time service of the wireless device.
 6. The method according toclaim 4, wherein the wake period of a wireless device is set based onthe QoS requirement of the real-time service of the wireless device. 7.The method according to claim 1, further comprising changing the powersave cycle lengths of one or more wireless devices having larger powersave cycle lengths to a smaller integer multiple of the system referencecycle when new wireless devices associate with the central device.
 8. Amethod for scheduling power save cycles of a wireless device,comprising: receiving from a central station scheduling data for asleep/wake cycle, wherein a sleep/wake cycle length is defined as aninteger multiple of a system reference cycle that is common for aplurality of wireless devices and that is set as a minimum value of adelay constraint on real-time service for each wireless device of saidplurality of wireless devices that has a real-time service, and whereinthe integer value is varied between wireless devices to form adifference between a power save cycle length for each wireless device;and waking and setting to sleep the appropriate circuits as a functionof the scheduling data using the predetermined sleep/wake cycle length.9. The method according to claim 8, wherein the power save cycle lengthis attributed as the maximum integer multiple of the system referencecycle under the constraint of being no larger than the maximum delayrequirement on real-time service of the wireless device.
 10. The methodaccording to claim 8, wherein the wake period of a wireless device isset based on the QoS requirement of the real-time service of thewireless device.
 11. The method according to claim 8, wherein thescheduling data changes the power save cycle length to a smaller integermultiple of the system reference cycle when other wireless devicesassociate with the central station.
 12. A central device for schedulingpower save cycles in a wireless network comprising a plurality ofwireless devices, said central device being configured to: determine asystem reference cycle as a minimum value of a delay constraint on areal-time service for each wireless device of said plurality of wirelessdevices that has a real-time service; attribute a length of a power savecycle to each wireless device, wherein each power save cycle length isan integer multiple of a system reference cycle and wherein the integervalue is varied to form a difference between the power save cycle lengthfor each of the plurality of wireless devices; assign a sleep period anda wake period within the power save cycle of each wireless device usingthe predetermined power save cycle length; and arrange the power savecycle of each wireless device to avoid collision of the wake period withthose of other wireless devices.
 13. The central device according toclaim 12, wherein the central device is configured to adjust thestarting time of the power save cycle of each wireless device to avoidcollision of the wake period with those of other wireless devices. 14.The central device according to claim 12, wherein one or more wirelessdevices have a service with a high priority.
 15. The central deviceaccording to claim 14, wherein a real-time service has the highpriority.
 16. The central device according to claim 14, wherein thepower save cycle length of a wireless device having real-time service isattributed as the maximum integer multiple of the system reference cycleunder the constraint of being no larger than the maximum delayrequirement on real-time service of the wireless device.
 17. The centraldevice according to claim 14, wherein the wake period of a wirelessdevice is set based on the QoS requirement of the real-time service ofthe wireless device.
 18. The central device according to claim 12,wherein the device is further configured to change the power save cyclelengths of one or more wireless devices having larger power save cyclelengths to a smaller integer multiple of the system reference cycle whennew wireless devices associate with the central device.